The invention relates to a method and apparatus for making highly densified sheets preferably including long fiber glass mats and thermoplastic materials by densification of a thermoplastic granulate material layer with a granulate layer/final thickness ratio of &gt;1.3:1.
It is known to soak glass, carbon, stone or mineral fiber mats or oriented fibers of organic duroplastic materials with molten thermoplastic material in order to produce therefrom usually plate-like semifinished products from which then various structural parts, for example, for lightweight cars, are made in molding presses. Such semifinished products have been known in the plastic processing industry under the designation Glas Mat Thermoplasts (GMT).
It is also known to spread granulate material of highly densified thermoplastic resin such as ethylvinylacetate or polyvinylchloride and melt it in double-band presses and density it to form floor and roof coverings. The state of the art for such products is described in EP 0046526.
The method according to the invention and the apparatus according to the invention will be described herein based on the example of glass mat thermoplastics (GMT) but the invention is not limited thereto. Other materials with a bonding matrix of thermoplastic polymers or duromeres with thermoplastic phases or duromere filter material mixtures such as urea glue coated wood chips for the manufacture of wood chip panels are within the scope of the present invention if their granulate layer/final thickness ratio, that is, the layer thickness before entering the gap between the inlet drums of the double-band press according to the invention and the final product thickness, exceeds the value of 1.3 and the final product is to have no air enclosures.
It is important that, during preparation of the thorough mixture of thermoplastic polymers and the fibers for the GMT, the fiber length and orientation is fully maintained during the impregnating procedure and that, on the other hand, the surfaces of the fibers are fully wetted by the molten thermoplastic material.
The state of the art is described, for example, by U.S. Pat. No. 955,956 and German Patent No. DP 2312816. In order to achieve full impregnation of the surfaces of the glass fibers for the continuous impregnation of the glass mats the prior art proposes to combine the mats, upon approaching the inlet drums of the double-band press, with cover films of thermoplastic material or to deposit such films on the press bands and to feed the reinforcement fibers as a sheet-like fleece between the films.
In the reaction zone of the double-band press these thermoplastic films and the fiber mats and a melt coating applied for example by extrusion are heated to such temperatures and densified under such pressures which result in the properties required for the subsequent use.
A well-known disadvantage of this process which, with the progress in manufacturing techniques, is increasingly objectionable resides in the high manufacturing costs for the cover sheets produced in a separate process and the complicated machinery required for feeding and guiding the sheets into the reaction zone. It is also disadvantageous that the sheet portion of the poly matrix has to be twice subjected to high temperatures.
Another disadvantage of GMT produced in accordance with the state of the art is the substantial content of air enclosed with the impregnation step which inhibits even warm-up in hot air ovens as a result of cavity formation and swelling up of the section being prepared for further processing.
It is therefore an objective to obtain semifinished GMT products with a density of up to 99% of the theoretical value, that is, which came close to the theoretical value by up to 0.1%, at which there are no air enclosures and each reinforcement fiber is enclosed in the resin matrix and wherein there is furthermore only a minimum amount of fiber breakage.
It is further desirable to provide for a higher fiber concentration in the outer layers in order to improve the flow characteristics during molding and the rigidity of the molded parts formed from the GMT.
It has been proposed in German Patent No. 3,347,877 to provide GMT in the form of a fabric to supply the resin matrix solely in the form of a solid sheet and to melt such package together in the reaction zone. In this process the reaction zone is limited along its sides by calibrated sealing wires capable of withstanding pressure whereas the still solid raw material package closes the inlet end and the already solidified GMT tightly closes the outlet end.
Although this process provides for GMT of very high density it requires twice the thermal conversion of the resin matrix which, in addition to submitting the resin of which the matrix consists to double thermal exposure, is also quite expensive. Furthermore, the length of the double-band press in relation to the GMT output per time unit is quite large because of the time required for heating the base materials to their melting temperature and the process is also for that reason relatively expensive.